1. Field of the Invention
The present invention relates to wireless communication and more particularly, a method for transmitting a frame using multiple physical layers in a wireless LAN system.
2. Related Art
Along with the advancement of information and communication technologies, various kinds of wireless communication technologies are under development. Of these, wireless local area network (WLAN) technology, which utilizes wireless frequency technology, enables users to connect to the Internet wirelessly at their home, their workplace, or at a particular service area by using their portable device such as a personal digital assistant (PDA), a laptop computer, and a portable multimedia player (PMP).
Since the IEEE (Institute of Electrical and Electronics Engineers).802, which is a standards developing organization (SDO) for WLAN technology, was established on February, 1980, a lot of standardization tasks are being carried out. The initial WLAN specification supported a speed of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a frequency of 2.4 GHz according to the IEEE 802.11 standards, and recent specifications are capable of supporting a maximum speed of 54 Mbps by using Orthogonal Frequency Division Multiplexing (OFDM). In addition, the IEEE 802.11 working group is developing standards of various technologies such as improvement of Quality of Service (QoS), Access Point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access vehicular environments, fast roaming, a mesh network, interworking with an external network, and wireless network management.
The basic access mechanism of the IEEE 802.11 MAC (Medium Access Mechanism) uses the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mechanism combined with a binary exponential backoff. The CSMA/CA mechanism is also called a distributed coordination function (DCF) of the IEEE 802.11 MAC and adopts the “listen before talk” mechanism by default. In this type of connection mechanism, a station (STA) listens to a radio channel or a medium prior to starting transmission. As a result of the listening, if it is found that the medium is not in use, the listening station starts its own transmission. On the other hand, if the medium is in use, the station enters a delay period determined by the binary exponential backoff algorithm rather than start its own transmission.
The CSMA/CA mechanism supports virtual carrier sensing in addition to physical carrier sensing whereby the station listens to a medium directly. Virtual carrier sensing is intended to compensate for limitations of physical carrier sensing such as a hidden node problem. To implement virtual carrier sensing, the IEEE 802.11 MAC (Medium Access Control) employs a network allocation vector (NAV). Suppose an STA currently uses a medium or has a right to use the medium. The STA then makes use of the NAV to inform the other STAs about the remaining time before the medium is available again. Therefore, a value carried by the NAV corresponds to a time period during which the medium is supposed to be used by the STA which transmits the corresponding frame.
Configuration of the NAV requires a procedure of exchanging RTS (Request To Send) frame and CTS (Clear To Send) frame. The RTS and CTS frame contain information which informs reception STAs about upcoming frame transmission and which can be used for delaying transmission of frames by the reception STAs. The information can be carried by, for example a duration field of the RTS and the CTS frame. Once exchange of the RTS and the CTS frame is carried out, a source STA transmits an actual frame to a destination STA.